Are grazing increaser species better tolerators than decreasers? An experimental assessment of defoliation tolerance in eight British grassland species

1 The importance of tolerance to herbivory for plant survival has long been recognized, but capacity for regrowth following recurrent defoliation has rarely been studied. 2 A glasshouse experiment was conducted with eight grassland species. We chose both species favoured under herbivory (increasers) and those that become scarce (decreasers) and manipulated the ontogeny of the plant when first clipped, as well as frequency (up to eight times) and intensity of clipping. 3 Plant survival was high (88%) but both frequency and intensity of clipping increased plant mortality in all species investigated (P < 0.05). Immature plants showed the highest mortality (15.9%). 4 Plant biomass mirrored mortality with immature plants again the most affected. As expected, control plants were largest and plants experiencing 100% biomass removal smallest. Clipping frequency was also significant, but the effect was not linear and plants that were clipped more than twice were more dramatically affected. 5 Biomass compensation showed species-specific responses that were significantly related to an a priori definition of species status in the grassland: increaser species had significantly higher compensation ability after recurrent defoliation than decreaser species. 6 Tolerance to herbivory has been viewed as a marginal strategy for plant survival, but our results suggest that it plays an important role in explaining abundance and distribution of plant species in environments with recurrent defoliation. 7 Plants do not always respond linearly to environmental pressures (such as repeated defoliation) and cannot therefore be predicted by studies considering only the lower end of the possible intensities. Understanding the effects of environmental pressures on plant fitness requires the study of a broader range of intensities, at different ontogenic stages and consideration of possible response thresholds.